Atrial premature beats are excitations of the heart initiated by the atrium and are prematurely triggered; ventricular premature beats refer to heart excitations initiated by the ventricles, which are also premature. On the electrocardiogram (ECG), atrial premature beats can be seen as prematurely occurring P waves and QRS complexes, where the shape of the QRS complex is consistent with that of the normal sinus rhythm. In contrast, ventricular premature beats on the ECG are shown as prematurely occurring wide and abnormal QRS complexes, with no preceding P wave.